The term “in-situ stress in rock” refers to stress that exist in the interior of rock, including gravitational stress, tectonic stress, and residual stress. Here, gravitational stress indicates stress generated by the rock's own weight. Tectonic stress indicates stress generated by movement of the earth's crust. Residual stress indicates stress remaining after removal of its original cause, such as expansion or heating of the rock or a past surface load since removed by surface erosion.
In the design or safety analysis of a large-scale structure in rock such as a tunnel or an oil storage tank, calculation of in-situ stress in the rock is proving to be very important. This is because no underground structure can be designed and constructed in a stable and economical manner inside rock until the direction and magnitude of stresses acting on the rock have been accurately measured. For example, in tunnel construction, if pressure applied to surrounding rock is isotropic, the tunnel cross section is generally circular. On condition that there is strong transverse pressure on the rock such as by a surface load, though the tunnel cross section is elliptical, the tunnel keep safe from collapse.
For instance, when excavating a tunnel without exactly measuring the in-situ stress in the rock, the rock may become overstressed due to stress concentration on an excavated surface and may collapse or become unstable due to expansion of existing cracks. Thus, in order to install a structure in rock, an accurate measurement of in-situ stress in rock is required.
Methods for measuring in-situ stress in rock include hydraulic fracturing (Fairhurst, 1964), stress opening such as overcoring (Leeman and Hayes, 1966; Merrill, 1967; U.S. Pat. No. 4,491,022), an indirect method such as acoustic emission and so forth.
Among these methods, hydraulic fracturing and overcoring are frequently used. A conventional hydraulic fracturing system is illustrated in FIG. 1. Referring to FIG. 1, a borehole h is formed in the rock in which an underground structure will be constructed, and then packers 1 are installed at upper and lower ends of a test section a, thereby sealing the test section. Afterwards, a high-pressure pump (not shown) applies hydraulic pressure, thereby injecting fluid into test section a through a pipe p. When hydraulic pressure continuously increases to reach ‘initial fracturing pressure’, cracks are formed in a borehole wall w. When fluid is continuously injected, the cracks are gradually widened and pressure is lowered. When the injection is stopped, the cracks are closed and the pressure reaches ‘crack closing pressure’. Here, since the crack closure pressure is equal to or slightly greater than the pressure required to maintain the cracks, it represents the minimum principal stress, which acts in a direction perpendicular to the cracked surface. Further, while the above-mentioned processes are repeated (second cycle), the cracks are re-opened (crack re-opening pressure). Thus, after crack re-opening pressure is measured, the maximum horizontal principal stress can be calculated using the measured pressure.
Among the methods for measuring in-situ stress in rock, hydraulic fracturing has an advantage in that it can be applied to deep underground rock as long as the borehole can be drilled, but has a disadvantage in that it is not easily applied to specific types of rock, for instance, sedimentary rock with a stratified structure. Further, when pressure is applied using packers 1, the borehole may collapse or the packers may jam. This complicates withdrawal of equipment, especially at greater depths. Further, since the size of the equipment must be increased in order to apply high hydraulic pressure, there are difficulties in using the conventional hydraulic fracturing system.
As described above, hydraulic fracturing has a limitation in that only two parts of principal stress can be measured. In other words, hydraulic fracturing has a limitation in that, since the vertical stress is set to the surface load (density*mass*height), the maximum and minimum principal stress can be measured in only a horizontal direction, and thus in-situ stress cannot be accurately measured. Furthermore, many researchers who have extensively studied hydraulic fracturing cast a doubt on the accuracy of crack re-opening pressure as well as the variation in pore hydraulic pressure in cracks (Ito et al., 2001).
Unlike hydraulic fracturing, overcoring has an advantage in that six stress components existing on three dimensions can be provided. However, since overcoring is based on the measurement of strain, it requires elaborate dual coring work. Hence, when applied, overcoring is problematic in that it is highly complex as well as restricted to the depth of the borehole.